The invention relates to the moving-bed processes such as gasoline reforming (reforming according to English terminology) for the improvement of the octane number of petroleum fractions that are located in the range of gasolines, i.e., at the starting point between 70° C. and 110° C., and at the end boiling point between 150° C. and 180° C. More generally, the invention relates to any process that uses a series of moving-bed reactors such as are found in the aromatizing process or the process for dehydrogenating normal paraffins. The following description refers to the reforming process with continuous regeneration that will be designated by the name of regenerative reforming. The process for reforming gasolines started in the 1950s and has since known important technological developments that are often linked to the appearance of new generations of catalysts according to three successive stages. The appearance of catalyst was based on platinum on alumina in the 1950s. The units worked at pressures on the order of 5 MPa, and the catalyst was regenerated about every 6 months. Toward the end of the 1960s, the bimetallic catalysts that would allow the operating pressure to be lowered to around approximately 3 MPa appeared. Finally, at the beginning of the 1970s, the appearance of the continuous regeneration of catalyst made it possible to reach operating pressures only on the order of 1 MPa.
Currently, the regenerative reforming units operate at pressures of several bar (1 bar=0.1 MPa), typically 3 bar (0.3 MPa), on very selective catalysts that produce maximum hydrogen and on feedstocks that have a tendency of becoming more narrow.
The general tendency that emerges from this development is the continuous reduction of the pressure whose impact on the reformate yields is very significant.
The chemical reactions that are involved in the reforming process are numerous. The main one among them is the dehydrogenation of naphthenes into aromatic compounds, which is the most desired chemical family, since it is the one that promotes high octane numbers. The dehydrocyclization of paraffins into aromatic compounds and the isomerization of paraffins and particularly paraffins with a carbon atom number of 5 or 6 are also desired, since they also accompany an increase in the octane number. Among the unfavorable reactions, i.e., that do not lead to an improvement of the octane number, it is possible to cite the hydrocracking of paraffins and naphthenes.
The thermodynamic data show that the equilibrium of various chemical families is shifted toward the low-pressure aromatic compounds, which explains the technological development of units toward increasingly weaker operating pressures, while maintaining a certain partial hydrogen pressure that makes it possible to limit the deactivation of the catalyst by the coke. Actually, the coke is a compound of high molecular weight, characterized by a low H/C ratio, generally between 0.3 and 1.0, which is deposited on the active sites of the catalyst. Although the transformation selectivity of the hydrocarbons into coke is very low, the contents of coke accumulated on the catalyst can be very significant. Typically, for the units in a moving bed, these contents are between 3 and 10% by weight at the outlet of the last reactor.
The current technology of the catalytic reforming units is that of the moving bed that appeared in the 1970s in two main forms: the one that is described in U.S. Pat. No. 4,119,526 that is characterized by a vertical stacking of reactors through which the feedstock passes in succession, and that of the applicant that is characterized by reactors that are placed side by side.
In the two cases, the effluents that are obtained from a reactor are heated in a furnace before being introduced at the top of the next reactor since, overall, the reactions that are involved are endothermic and the reactors are operated at the same starting temperature. In the technology with vertical stacking of the reactors, the catalyst flows by gravity from one reactor to the next, then it is picked up by a lift line or a pneumatic transport line at the outlet of the last reactor to be introduced at the top of the regenerator in which it also flows by gravity. At the bottom of the regenerator, it is picked up by a second lift line to be introduced at the top of the first reactor.
In the technology of the applicant, the reactors are placed side by side, and the catalyst also flows by gravity inside each reactor and is transported from the bottom of a reactor to the top of the next reactor via a lift line. It is picked up by a lift line at the bottom of the last reactor to be introduced at the top of the regenerator in which it also flows by gravity. At the bottom of the regenerator, the catalyst is also picked up by a lift line to be introduced at the top of the first reactor. A detailed description of the circulation of the catalyst is indicated in Patent FR 2 657 087.
In short, in the current technology of the regenerative reforming reactors with moving beds, the catalyst circulates in succession from one reactor to the next, the same as the intermediate effluents that pass from one reactor to the next. This means that the catalyst that enters a reactor, other than the first, is a catalyst that is already coked by the reactions that have taken place in the preceding reactors. A loss in catalytic activity, penalizing the operation of each reactor and causing the following reactors to be operated at a higher temperature than the one that could be used if there were no coke deposition on the catalyst that enters the reactors, results therefrom. At the outlet of each reactor, the effluents are sent into a reheating furnace so as to enter the next reactor at a temperature level that is generally essentially identical to that of the preceding reactor, but sometimes slightly different if the operation of the unit thus requires.
The typical operating conditions of a moving-bed reforming unit are as follows: operating pressure of between 0.3 MPa and 0.8 MPa, volumetric flow rate, i.e., ratio between the catalyst mass in a reactor and mass rate of the feedstock, between 1 and 4 h−1, molar ratio of hydrogen to hydrocarbons (H2/HC) of between 3 and 10, and more particularly between 3 and 5, mean starting temperature of the reactors of between 480 and 550° C.
This invention makes it possible to redefine the circulation of the catalyst so that the catalyst that enters all or part of the successive reactors is a catalyst that is at least partially regenerated, regardless of the technology that is used for the arrangement of the reactors. This invention can therefore be used with a technology of reactors placed side by side, or a technology of reactors that are stacked vertically on one another. In other words, the circulation of the catalyst becomes at least partially a circulation in parallel relative to the reactors, whereas that of the feedstock and effluents continues to be in series. Various embodiments of this circulation type will be presented in the text below.